1. Field of the Invention
The present invention relates to microstrip antennas and, in particular, to a method of enhancing the bandwidth of a microstrip antenna without increasing the size or weight of the antenna.
2. Description of the Related Art
Microstrip antennas have many interesting properties such as low profile and lightweight. However, the inherent narrow bandwidth of a microstrip antenna is one of its serious disadvantages. The conventional microstrip antenna typically exhibits a bandwidth of only 1-2% of the resonant frequency. The narrow bandwidth of the microstrip antenna is often inadequate to meet the requirements for practical applications. The development of techniques for the enhancement of the bandwidth of microstrip antenna has been a topic of special emphasis for several years.
A conventional microstrip antenna is shown in FIGS. 17A and 17B. The microstrip antenna 170 illustrated in FIGS. 17A and 17B consists of a dielectric substrate 101, a radiating element 102 constructed on the top surface of the substrate 101 and a ground plane 103 constructed on the bottom surface of the substrate 101. A power feed hole 104 is provided at a point corresponding to the radiating element 102 on the substrate 101. A connector 105, used for feeding radio frequency (RF) power to the radiating element 102, is inserted through the feed hole 104 from the bottom surface of the substrate 101. The connector 105 is electrically connected to the radiating element 102 with solder 106a and is fixed to the ground plane 103 by solder 106b.
The techniques currently available for enhancing the bandwidth of microstrip antennas (MSA) include use of a thicker substrate, multi-layer stacked microstrip antennas, electromagnetically coupled (EMC) microstrip antennas, microstrip antennas with parasitic elements, aperture coupled microstrip antennas, and use of external matching circuits. As will be clear from the explanations to be provided, some of the above techniques result in an increase in size and weight of the microstrip antenna while some others suffer from the lack in the structural simplicity usually associated with conventional microstrip antennas.
The prior art structural configurations of microstrip antenna for the improvement of bandwidth using the above mentioned techniques are described below. The elements of new microstrip antennas which are similar to that of the conventional microstrip antenna 170 will have same reference numbers as in FIGS. 17A and 17B and additional reference explanations will be omitted.
The prior art microstrip antenna 120 with thick substrate material shown in FIGS. 12A and 12B has the undesirable characteristics of increased height and weight of the antenna. The thick substrate of the microstrip antenna shown in FIGS. 12A and 12B increases the dielectric loss and also increases the cost of the antenna. The thick substrate of the antenna of FIGS. 12A and 12B also causes the generation of surface waves and hence degrades the radiation pattern, which is not desirable.
The prior art microstrip antenna 130 with parasitic elements illustrated in FIG. 13 has two additional parasitic elements 107 adjacent to the radiating element 102. A narrow gap separates these parasitic elements 107 from the main radiating element 102. The microstrip antenna 130 has the disadvantages of increased length and weight.
FIG. 14 illustrates the configuration of a prior art electromagnetically coupled microstrip antenna 140. Antenna 140 has two substrates 101 placed one above the other. The bottom surface of the top substrate 101 does not have conductive film. There is a radiating element 102 on the top surface of the upper substrate 101 and a narrow microstrip line 108 on the top surface of the lower substrate 101 acts as a feed for the radiating element 102. The microstrip antenna 140 has the disadvantages of increased height, increased weight and higher cost.
A prior art microstrip antenna 150 with multi-layer stacked elements is illustrated in FIG. 15. Antenna 150 has two radiating microstrip elements 102, one on the top surface of upper substrate 101 and the other on the top surface of the middle substrate 101. The radiating elements 102 are stacked one above the other. A narrow microstrip line 108 is positioned on the top surface of bottom substrate 101. Microstrip line 108 serves as a common feed for the two radiating elements 102. As in microstrip antenna 140, there is no conductive film on the bottom surfaces of the upper and middle substrates 101. The disadvantages of microstrip antenna 150 are increased height, weight, complexity of design, and higher cost.
A prior art aperture coupled microstrip antenna 160 is shown in FIG. 16 and comprises a radiating element 102 on the top surface of upper substrate 101 and a conductive ground plane 103 with an opening or aperture 109. A narrow microstrip feed line 108 positioned on the top surface of bottom substrate 101 serves as a feed to the aperture 109. Power is coupled to the radiating element 102 through the aperture 109. The disadvantages of microstrip antenna 160 are structural complexity, design complexity, increased height, increased weight, and higher cost.
The prior art microstrip antenna with external matching circuit involving inductors and capacitors does not increase the height and or linear dimensions of the antenna. The inductors and capacitors are used near the feed point of the microstrip antenna and provide a better impedance match, hence an improvement in bandwidth results. The disadvantage is that increased bandwidth is at the expense of an undesirable loss in gain of the antenna. Although the matching circuit components are part of the device to which the microstrip antenna is attached and technically are not part of the antenna, they do add to the total cost of the device.
In the past, shorting pins or slots have been used in microstrip antennas to reduce the resonant frequency or to achieve a dual frequency mode of operation. In the prior art, slots or shorting pins have been used separately to achieve dual frequency performance of the antenna. See, for example, S. C. Pan and K. L. Wong "Design of Dual Frequency Microstrip Antennas using shorting pin loading", IEEE-APS Symposium, Atlanta, June 1998, pp. 312-315; K. L. Wong and W. S. Chen, "Compact microstrip antenna with dual-frequency operation", Electronics Letters, Apr. 10th 1997, Vol. 33, No. 8, pp. 646-647; S. Maci, Biffi Gentili, P. Piazzesi and C. Salvador, "Dual band slot-loaded patch antenna", IEE Proc.-Microw. Antennas Propag., Vol. 142, No. 3, June 1995, pp. 225-232; and S. Maci, G. Biffi Gentili and G. Avitabile, "Single-Layer Dual Frequency Patch Antenna", Electronics Letters, Aug. 5th 1993, Vol. 29, No. 16, pp. 1441-1443, hereinafter referred to as Pan et al., Wong et al., Maci et al., and Maci et al. (II), respectively.
B. F. Wang and Y. T. Lo, "Microstrip Antennas for Dual-Frequency Operation", IEEE Transactions on Antennas and Propagation, Vol. AP-32, No. 9, September 1984, pp. 938-943, describes the dual frequency operation of a microstrip antenna using a combination of slots and shorting pins. In the above-cited references, the obtained bandwidths centered around the dual resonant frequencies have been relatively narrow (1-2% of resonant frequencies). There is also a practical lower limit for ratio of (f.sub.u /f.sub.L) (f.sub.u and f.sub.L being the upper and lower resonant frequencies, respectively). As a consequence of the lower ratio of (f.sub.u /f.sub.L), the resonant bands centered around the dual resonant frequencies are rather widely separated. Therefore, combining the two narrow resonant bands to improve the overall bandwidth is very difficult using the previously used configurations that have been illustrated in the above references.
To circumvent the existing disadvantages of the available microstrip antenna bandwidth enhancing techniques, it is the objective of the present invention to design a single substrate microstrip antenna possessing structural simplicity, wider bandwidth, lightweight, compact size, ease of fabrication, and cost effective to manufacture.